Silacyclopropenylidenes

S-37 (see above) it is also possible to prepare and to matrix-isolate the silicon species 124, 125, and 126, which again exist in a photoequilibrium. Our first entry to 1-silacyclopropenylidene (124) was the pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane (123).71,72 Even though the structure of educt molecule 123 suggests formation of ethynylsilylene (125), the isolated product was 124. Obviously 125 had already thermally isomerized to the most stable isomer 124 before the products were condensed at 10 K. 

Srinivas, R. Stilzle, D. Weiske, T. Schwarz, H. Generation and Characterization of Neutral and Cationic 3-Silacyclopropenylidene in the Gas Phase. Description of a New BEBE Tandem Mass Spectrometer. Int. J. Mass Spectrom. lonProc. 1991,107, 369-376. 

Thermally generated silicon atoms react in an argon matrix with acetylene to give silacyclopropenylidene and in a similar way with ethylene to give silacyclopropylidene... 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Another example which clearly demonstrates the advantageous interplay between different experimental techniques and state-of-the-art ab initio methods is that of the ionic and neutral 3-silacyclopropenylidene (49) and its isomers 50-53170. 

A particle whose connectivities are indicative of structure 104 has earlier been detected by neutralization-reionization mass spectrometry134. Furthermore, 1-silacyclopropenylidene (104) was recently identified by microwave spectroscopy135. 1-Silacyclopropenylidene (104) (kmax = 286 nm136 a delocalized three-center jr-bond orbital128) generated from 102 and matrix-isolated is transformed into ethynylsilylene... 

Ethynylsilylene (103) (kmax = 500 nm136 calculated kmax = 520 nm137) isomerizes into 1-silacyclopropenylidene (104) and vinylidenesilylene (105) when irradiated with X = 500 nm127-129. Vinylidenesilylene (105) shows two weak infrared bands and a UV absorption with vibrational fine structure (X = 340, 325 and 310 nm136). Irradiation into this absorption leads back to ethynylsilylene (103) (equation 28)127-129. 

If the behavior of the related precursors 102, 144, 137 and 149 is compared (equation 42), it is remarkable that the two former yield the silacyclopropenylidenes 104 and 145 as the first detectable species, whereas the fragmentation of the methyl substituted compound 137 stops at the open-chain silylene 138. In the case of the diethynyl precursor 149 both products (150 and 151) are observed. 

The observed product distributions reflect the influence of both factors. In the parent compound 102 and in the silyl substituted 104, the initially formed open-chain silylenes 103 and 146 are relatively sensitive and the migration tendency of the group R is quite large, and therefore the silacyclopropenylidenes 104 and 145 are found. In contrast, the methyl and the ethynyl compound 138 and 150 are stable enough to pass the pyrolysis tube and both can be matrix-isolated upon pyrolysis of the appropriate precursors 137 and 149. Due to the small migration tendencies of the methyl and the ethynyl group, the rearrangement product 135 is not found at all and 151 is detected only in relatively minor amounts. 

Maier and coworkers have found that pulsed flash pyrolysis of an acetylenic disilane 323 gave rise to the acetylenic silylene 324, which subsequently rearranged to the cyclic silylene 1-silacyclopropenylidene 325162. Irradiation of this cyclic silylene resulted in its isomerization to the isomeric acetylenic silylene 324, which itself could be photo-chemically converted to the allenic silylene 326. As well, both 324 and 325 were said to isomerize on photolysis to the unusual silacycloalkyne 327, which was characterized... 

According to calculations by H. F. Schaefer et al. [1], 1-silacyclopropenylidene 3 is expected to be the most stable C2H2Si species. Thus, it has been discussed as the adduct of a Si atom with acetylene [2, 3]. 

Under these pyrolysis conditions the IR spectrum shows, apart from the bands for trimethylsilane (2), those of another compound. This compound is identified as 1-silacyclopropenylidene (3) - even though the structure of reactant 1 suggests formation of ethynylsilanediyl (ethynylsilylene ) (4) - by comparison with the IR spectra for 3, 4, and 5 calculated by ab initio methods [la]. Furthermore, 3 exhibits a weak, broad UV absorption between 320 and 260 nm 286 nm). 

It is therefore not surprising that irradiation of 4 with visible light of wavelength 500 nm mainly leads to reisomerization to give 1-silacyclopropenylidene (3). A small amount of a new species is also formed, and two weak IR bands at 1667.5 and 957.7 cm" and a UV band with fine structure (A = 340, 325, and 310 nm) are recorded. Irradiation into this absorption (X = 340 nm) leads to reisomerization to 4 hence, this species is another isomer of C2H2Si. By comparison with the calculated IR speetra this isomer is identified as vinylidenesilanediyl (5). 

Among the cyclic silylenes 4-7, SiC2 (4) [2] and 1-silacyclopropenylidene (5) [3] have already been investigated by matrix spectroscopy. This series is now completed by silylene 6, the formal hydrogenation product of 5. We also report on the isolation of 7, which can be derived from 5 by formal substitution of one CH unit by a nitrogen atom. 

As expected, the primary product of the cocondensation is 1-silacyclopropenylidene (5). The photochemical interconversion to the other known C2H2Si isomers could be reproduced this is shown in the scheme below for completion. 

Compound 11 can be easily transformed into 2-aza-l-silacyclopropenylidene (7, weak IR absorption for the C-N stretching vibration at 1466 cm" ), the cyclic analogue to 5 and 6 in the reactions described in the previous sections. The photoreaction requires only irradiation with visible light of wavelengths > 570 nm and is complete within a few minutes, indicative of a low activation barrier for this isomerization. In the course of irradiation, a small amount of another species is also formed. The IR absorptions of this second species increase upon irradiation with light of wavelengths > 395 nm, while those of 7 disappear. By comparison with calculated IR spectra, this species can be... 

Obviously silacyclobutenylidene 5 should be a suitable intermediate on the way to silacyclobutadiene (4), lying just 5.1 kcal/mol above the global minimum (silacyclopropenylidene 14 see Fig 1) and being separated from 4 only by a 1,2-hydrogen shift. This type of reaction is well-known in silylene chemistry. For example, methylsilylene (6) and silaethene (7) are readily interconvertible photochemically [5]. 

Analogous to the isolation of silacyclopropenylidene upon thermal trimethylsilane extrusion from 2-ethynyl-l,l,l-trimethyldisilane [6], pyrolysis of 8 could lead to silacyclobutenylidene 5. Therefore, propargyldisilane 8 and all other stable open chain isomers of 8 were prepared and examined. 

The pyrolysis of propynyldisilane 13 took the same course as that of l,l,l-trimethyl-2-ethynyldisilane. In the initial step, silacyclopropenylidene 14, the global minimum on the CsHtSi hypersurface, was formed. Irradiation with light of wavelength /I = 313 nm yielded propynylsilylene (15) Further irradiation at 254 nm resulted chiefly in the generation of ethynylmethylsilylene (12). This isomer is an important link between the chemistry of precursors 8, 10 and 13. Moreover, silylene 12 is the primary product in the pyrolysis of ethynyldisilane 16. The already known species 5 is observed after irradiation with 254 nm as well, but in this case only the most intense IR absorption was registered. 

Ab Initio calculations suggest Si=C of (Me Si)2Si C(Ad)OSiMe (176 pm) is electronically elongated and should be 170 pm long, diile substituents reduce the reactivity of Si C through reversed V-polarity. Silyl substituted silacyclopropenylidene is the most stable of 028128 cyclic compounds, while silene-silylene interconversions are thermoneutral, but with a high (24-58 heal) activation 18... 

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